1. Field of the Invention
The present invention relates to an attachment lens which is ideally suited for a silver salt photographic camera, a video camera, an electronic still camera, etc., which is installed on the image surface side of a master lens serving as a photographic optical system, and which extends the focal length of the system to a longer side. The present invention also relates to a lens assembly employing the attachment lens, and a camera system equipped with the lens assembly.
2. Description of the Related Art
There have been proposed a variety of rear attachment lenses installed on the image surface side of master lenses, which serve as photographic optical systems, to extend the focal lengths of the systems toward longer focal length side.
In most rear attachment lenses designed to produce no aberration, the residual aberration of the master lens increases as the magnification increases, leading to deterioration in image quality. For example, when the magnification is .times.2, the transverse aberration such as comatic aberration and lateral chromatic aberration is simply doubled, thus deteriorating image quality. Spherical aberration, curvature of field, and longitudinal aberration such as longitudinal chromatic aberration increase by the square of the magnification, that is, by four times. In the case of a rear attachment lens, since the F number of the master lens is also doubled, meaning increased darkness, the longitudinal aberration per unit focal depth is eventually doubled with resultant deterioration in image quality.
Of the residual aberrations of the master lens, the spherical aberration, the comatic aberration, the curvature of field, the longitudinal chromatic aberration, and the like could be inversely corrected by increasing the number of lens elements constituting the rear attachment lens. In principle, however, it is difficult to correct the lateral chromatic aberration due to the following reasons: the signs of the heights of the axial paraxial rays and pupil paraxial rays, which enter the rear attachment lens, from an optical axis are always the same, the rear attachment lens essentially has a relatively large negative refractive power (1/focal length) as a whole, and the Abbe numbers of existing vitric materials are all positive values.
Especially in the case of a telescopic lens assembly which frequently employs a rear attachment lens, the chromatic aberration tends to worsen as the focal length increases. When the rear attachment lens is installed, the lateral chromatic aberration, which is increased and is difficult to be corrected as mentioned above, becomes a chief cause for deteriorated image quality. No rear attachment lens with a satisfactorily corrected lateral chromatic aberration has yet been realized.